Femtosecond filamentation in transparent media

There is growing interest in states of matter with topological order. These are characterized by highly stable ground states robust to perturbations that preserve the topology, and which support excitations with so-called anyonic statistics. Topologically ordered states can arise in two-dimensional lattice-spin models, which were proposed as the basis for a new class of quantum computation. Here, we show that the relevant hamiltonians for such spin lattice models can be systematically engineered with polar molecules stored in optical lattices, where the spin is represented by a single-valence electron of a heteronuclear molecule. The combination of microwave excitation with dipole–dipole interactions and spin–rotation couplings enables building a complete toolbox for effective two-spin interactions with designable range, spatial anisotropy and coupling strengths significantly larger than relevant decoherence rates. Finally, we illustrate two models: one with an energy gap providing for error-resilient qubit encoding, and another leading to topologically protected quantum memory.

https://arxiv.org/pdf/quant-ph/0512222.pdf

A toolbox for lattice-spin models with polar molecules

The authors review progress in understanding the nature of atomic collisions occurring at temperatures ranging from the millidegrees Kelvin to the nanodegrees Kelvin regime. The review includes advances in experiments with atom beams, light traps, and purely magnetic traps. Semiclassical and fully quantal theories are described and their appropriate applicability assessed. The review divides the subject into two principal categories: collisions in the presence of one or more light fields and ground-state collisions in the dark.

Experiments and theory in cold and ultracold collisions

Photoassociation is the process in which two colliding atoms absorb a photon to form an excited molecule. The development of laser-cooling techniques for producing gases at ultracold !!1 mK" temperatures allows photoassociation spectroscopy to be performed with very high spectral resolution. Of particular interest is the investigation of molecular states whose properties can be related, with high precision, to the properties of their constituent atoms with the “complications” of chemical binding accounted for by a few parameters. These include bound long-range or purely long-range vibrational states in which two atoms spend most or all of their time at large internuclear separations. Low-energy atomic scattering states also share this characteristic. Photoassociation techniques have made important contributions to the study of all of these. This review describes what is special about photoassociation spectroscopy at ultracold temperatures, how it is performed, and a sampling of results including the determination of scattering lengths, their control via optical Feshbach resonances, precision determinations of atomic lifetimes from molecular spectra, limits on photoassociation rates in a Bose-Einstein condensate, and briefly, production of cold molecules. Discussions are illustrated with examples on alkali-metal atoms as well as other species. Progress in the field is already past the point where this review can be exhaustive, but an introduction is provided on the capabilities of photoassociation spectroscopy and the techniques presently in use.

/pdf/ultracold-photoassociation-spectroscopy-long-range-molecules-31d6x6vqxx.pdf

Ultracold photoassociation spectroscopy: Long-range molecules and atomic scattering

Summary: Carbazole-based oligomeric and polymeric materials have been studied for almost 25 years for their unique electrical, electrochemical and optical properties. Interestingly, carbazole units can be linked in two different ways leading to either poly(3,6-carbazole) or poly(2,7-carbazole) derivatives. While the former class seems to be very interesting for electrochemical and phosphorescence applications, the latter shows very promising optical properties in the visible range for light emitting diodes (LED). The major intrinsic difference between these two classes is the effective conjugation length: poly(2,7-carbazole) materials having the longer one, due to their poly(p-phenylene)-like structure. Using different synthetic strategies and substitution patterns, the physico-chemical properties of both classes can be fine-tuned, leading to high performance materials for a large number electronic applications.

Chemical structures for poly(3,6-carbazole) and poly(2,7-carbazole) and the materials used as the starting points for their respective syntheses.

Polycarbazoles: 25 Years of Progress

We determined new Dunham coefficients of the A1∑+-State of NaH and NaD from degenerate Four- Wave-Mixing (DFWM) spectra in the near UV and in the blue spectral region. In the case of NaH we combined these data with results of Rafi et al. and Orth et al.. The new set of coefficients describes the vibrational dependence of the rotational constants B and D and of the band origins from ύ = 0 up to 25. The spectral positions of the lines in our DFWM-spectra can be reproduced by this coefficients with an accuracy better than 0.3 cm-1 for J-values ≤ 15 and 2 cm-1 for J-values ≤ 25. Especially for high J-values this is an improvement up to 30 times in comparison to Dunham coefficients recommended before. A RKR-potential of the A1∑+-state was calculated with the new coefficients. The Dunham coefficients of NaD were obtained by scaling the NaH coefficients with the reduced masses of the molecules. A comparison of our results to molecular constants determined from the measured NaD-spectra shows good agreement.

New Dunham coefficients of the A1∑+-State of NaH and NaD

In present experiments gradual change from the frequency comb excitation to pulse by pulse excitation of Rb atoms is observed. Shown results could lead to the development of a new method for system coherence monitoring.

/pdf/cancellation-of-the-coherent-accumulation-in-rubidium-atoms-4edrtipka5.pdf

Cancellation of the coherent accumulation in rubidium atoms excited by a train of femtosecond pulses

Excited LiZn and LiCd molecules are produced in a photochemical reaction between Li2(C1Πu) molecules and zinc or cadmium atoms. Changes in shape of the blue-green emission are observed by increasing the total vapor pressure in the heat pipe oven. Quantum mechanical spectral simulations are carried out for the relevant 22Π →X2Σ+ transitions. Both bound-bound and bound-free types of transitions are found to play an important role in the overall blue-green spectra. For high collision frequencies (high total vapor pressure) the simulated spectra show very good agreement with observations if a thermal distribution in the 22Π states is assumed. In this case bound-bound transitions are dominant. The results for low collision frequencies indicate that the nascent distribution is non-Boltzmann and that bound-free transitions are dominant.

Spectral simulation and interpretation of LiZn and LiCd blue-green emission

Discrete tuning of the single longitudinal mode over the gain profile of the 457.9 nm Ar-ion laser line has been used in order to directly excite the C1 Pi u and 21 Sigma +u states. The authors have observed fluorescence spectra of the interference continuum (21 Sigma +u-X1 Sigma +g) and the C-X molecular band together with collisionally induced diffuse-band emission (23 Pi g-13 Sigma +u). The excitation processes leading to the observed emission are discussed.

Goran Pichler

Papers

New Dunham coefficients of the A1∑+-State of NaH and NaD

Cancellation of the coherent accumulation in rubidium atoms excited by a train of femtosecond pulses

Spectral simulation and interpretation of LiZn and LiCd blue-green emission

A study of structured continua in K2 excited by the 457.9 nm Ar-ion laser line

Energy transfer in Li–Cd vapor mixture: )